Outreach programs to improve life circumstances and prevent further adverse developmental trajectories of at‐risk youth in OECD countries: A systematic review

Abstract Background At‐risk youth may be defined as a diverse group of young people in unstable life circumstances, who are currently experiencing or are at risk of developing one or more serious problems. At‐risk youth are often very unlikely to seek out help for themselves within the established venues, as their adverse developmental trajectories have installed a lack of trust in authorities such as child protection agencies and social workers. To help this population, a number of outreach programmes have been established seeking to help the young people on an ad hoc basis, meaning that the interventions are designed to fit the individual needs of each young person rather than as a one‐size‐fits‐all treatment model. The intervention in this review is targeted outreach work which may be (but does not have to be) multicomponent programmes in which outreach may be combined with other services. Objectives The main objective of this review was to answer the following research questions: What are the effects of outreach programmes on problem/high‐risk behaviour of young people between 8 and 25 years of age living in OECD countries? Are they less likely to experience an adverse outcome such as school failure or drop‐out, runaway and homelessness, substance and/or alcohol abuse, unemployment, long‐term poverty, delinquency and more serious criminal behaviour? Search Methods We identified relevant studies through electronic searches of bibliographic databases, governmental and grey literature repositories, hand search in specific targeted journals, citation tracking, and Internet search engines. The database searches were carried out in September 2020 and other resources were searched in October and November 2021. We searched to identify both published and unpublished literature, and reference lists of included studies and relevant reviews were searched. Selection Criteria The intervention was targeted outreach work which may have been combined with other services. Young people between 8 and 25 years of age living in OECD countries, who either have experienced or is at‐risk of experiencing an adverse outcome were eligible. Our primary focus was on measures of problem/high‐risk behaviour and a secondary focus was on social and emotional outcomes. All study designs that used a well‐defined control group were eligible for inclusion. Studies that utilised qualitative approaches were not included. Data Collection and Analysis The total number of potentially relevant studies constituted 17,659 hits. A total of 16 studies (17 different interventions) met the inclusion criteria. Only five studies could be used in the data synthesis. Eight studies could not be used in the data synthesis as they were judged to have critical risk of bias and, in accordance with the protocol, were excluded from the meta‐analysis on the basis that they would be more likely to mislead than inform. Two studies (three interventions) did not provide enough information enabling us to calculate an effect size and standard error, and one study did not provide enough information to assess risk of bias. Meta‐analysis of all outcomes were conducted on each conceptual outcome separately. All analyses were inverse variance weighted using random effects statistical models incorporating both the sampling variance and between study variance components into the study level weights. Random effects weighted mean effect sizes were calculated using 95% confidence intervals. Too few studies were included to carry out any sensitivity analyses. Main Results Four of the five studies used for meta analysis were from the USA and one was from Canada. The timespan in which included studies were carried out was 32 years, from 1985 to 2017; on average the intervention year was 2005. The average number of participants in the analysed interventions was 116, ranging from 30 to 346 and the average number of controls was 81, ranging from 32 to 321. At most, the results from two studies could be pooled in a single meta‐analysis. It was only possible to pool the outcomes drug (other than marijuana) use, marijuana use and alcohol use each at two different time points (one and 3 months follow up). At 1 month follow up the weighted averages varied between zero and 0.05 and at 3 months follow up between −0.17 and 0.07. None of them were statistically significant. In addition, a number of other outcomes were reported in a single study only. Authors' Conclusions Overall, there were too few studies included in any of the meta‐analyses in order for us to draw any conclusion concerning the effectiveness of outreach. The vast majority of studies were undertaken in the USA. The dominance of the USA as the main country in which outreach interventions meeting our inclusion criteria have been evaluated using rigorous methods and within our specific parameters clearly limits the generalisability of the findings. None of the studies, however, was considered to be of overall high quality in our risk of bias assessment and the process of excluding studies with critical risk of bias from the meta‐analysis applied in this review left us with only five of a total of 16 possible studies to synthesise. Further, because too few studies reported results on the same type of outcome at most two studies could be combined in a particular meta‐analysis. Given the limited number of rigorous studies available from countries other than the USA, it would be natural to consider conducting a series of randomised controlled trials evaluating the effectiveness of outreach for at‐risk youth in countries outside the USA. The trial(s) should be designed, conducted and reported according to methodological criteria for rigour in respect of internal and external validity to achieve robust results and preferably reporting a larger number of outcomes.

1 | PLAIN LANGUAGE SUMMARY 1.1 | Evidence on the effectiveness of outreach programmes for at-risk youth in OECD countries is inconclusive The evidence on outreach programmes to improve life circumstances and prevent further adverse developmental trajectories of at-risk youth in OECD countries is inconclusive.
In this review, we aimed to find evidence of the effectiveness of outreach programmes on improving at-risk youth's life circumstances.
However, the evidence is inconclusive because of the small number of studies.

| What is this review about?
At-risk youth are defined as a diverse group of young people in unstable life circumstances, who are currently experiencing, or at risk of developing, one or more serious problems. At-risk youth are often very unlikely to seek out help for themselves within the established facilities, as their adverse developmental trajectories have installed a lack of trust in authorities.
A number of outreach programmes have been established seeking to help these young people on an ad hoc basis, meaning that the interventions are designed to fit the individual needs of each young person rather than as a one-size-fits-all treatment model.

What is the aim of this review?
This Campbell systematic review examines the effects of outreach programmes on problem/high-risk behaviour of young people between eight and 25 year old, living in OECD countries. The review summarises evidence from five studies undertaken in the USA and Canada that involved 578 participants in total.

| What studies are included?
Included studies had to examine the impact of targeted outreach programmes on at-risk youth. Studies had to have a comparison group.
Sixteen studies analysing 17 different interventions were identified. Of these, only five studies could be used in the data synthesis. The studies were from the USA and Canada. There were four randomised controlled trials (RCTs) and 12 nonrandomised studies. The studies contained data for 578 participants.
1.4 | What are the main findings of this review?
The evidence was inconclusive. At most, the results from two studies could be pooled in a single meta-analysis. The outcomes drug (other than marijuana) use, marijuana use and alcohol use each at two different time points (one and three months follow up) were metaanalysed. In addition, a number of other outcomes were reported in a single study only.
1.5 | What do the findings of the review mean?
The current landscape of research on outreach programmes targeting at-risk youth in the OECD countries shows that it has yet to be evaluated thoroughly. The evidence was inconclusive because too few studies reported results on the same type of outcome.
Furthermore, all the available evidence used in the data synthesis was from the USA and Canada, and so the findings may not be generalisable to other settings and systems outside Northern America.
None of the studies used in the meta-analyses reported on long term impacts.
These considerations point to the need for more rigorouslyconducted studies reporting a larger number of outcomes.

| How up-to-date is this review?
The review authors searched for studies published up to November 2021.
2 | BACKGROUND 2.1 | Description of the condition At-risk youth may be defined as a diverse group of young people in unstable life circumstances, who are currently experiencing or are at risk of developing one or more serious problems such as school failure or drop-out, mental health disorders, substance and/or alcohol abuse, unemployment, long-term poverty, delinquency and more serious criminal behaviour (Arbreton et al., 2005;Quinn, 1999). Atrisk youth typically have a multitude of social and psychological problems and typically also come from families considered at-risk (Treskon, 2016). They may occasionally or permanently be homeless and spend time in the streets.
No readily available statistics on the numbers of at-risk youth exist but statistics on the numbers experiencing the adverse outcomes can be found. For example, according to the National Conference of State Legislatures (NCSL) on any given night, approximately 41,000 unaccompanied youth ages 13-25 experience homelessness in the US (NCSL, 2019). It is estimated that 4.2 million youth and young adults experience homelessness each year, and that FILGES ET AL. | 3 of 28 10% of young adults ages 18-25, and at least one in 30 adolescents ages 13-17, experience some form of homelessness over the course of a year (NCSL, 2019). A substantial part of them report having a number of other problems too; for example having substance misuse problems (29%), mental health problems (69%) or been in the juvenile justice system, in jail or detention (50%), Further, school drop-out and no high school diploma or General Equivalency Diploma (GED) is the number one correlate for elevated risk of youth homelessness (NCSL, 2019). In Denmark the numbers are much lower. The estimated number of homeless youth, less than 25 years of age, was 1036 in 2019 (Benjaminsen, 2019) which amounts to less than 1% of those aged 13-24 years; but in line with the evidence from the US a large part of them have other problems (e.g., substance misuse and mental health problems) as well and the majority in the age group 18-24 are NEET, that is, neither employed nor in education or training (Benjaminsen et al., 2020). Numbers of homeless youth across OECD countries are hard to locate and definitions of homelessness vary across countries (OECD, 2020a) but most likely, there is as great variation as in other indicators of at-risk youth. For example, the rates of school drop-out, those that do not reach a basic minimum level of skills, is on average 19% across OECD countries and range from 2% in Korea to 58% in Turkey for the 25-34 yearsold (OECD, 2012). Also, the NEET rates vary a lot across OECD countries; from less than 7% of the 15-29 year old in Iceland and the Netherlands to more than 37% in South Africa with an OECD average of 13% (OECD, 2020b).
At-risk youth are often very unlikely to seek out help for themselves within the established venues, as their adverse developmental trajectories have installed a lack of trust in authorities such as child protection agencies and social workers (Ronel, 2006). To help this population, a number of outreach programmes have been established seeking to help the young people on an ad hoc basis, meaning that the interventions are designed to fit the individual needs of each young person rather than as a one-size-fits-all treatment model (Korf et al., 1999;Svensson, 2003). The programmes are often multicomponent interventions and often rely on volunteers as outreach workers, as these are proposed to offer the young people a unique possibility for forming trusting relationships due to the fact that help is offered as an act of altruism (Ronel, 2006).
The programmes may offer basic necessities such as food or shelter, and they may offer counselling, mentoring and medical assistance.
What define the outreach programmes is that they are targeted at helping the young people away from the streets and their current adverse developmental paths towards more stable living situations and developmental prospects.
Due to the very nature of the programmes, the effects are difficult to determine. First, randomisation is difficult when there is no system of referral, and the uniquely tailored interventions, which each young person receives raises the question if one can even describe the intervention as uniform even within the same programme. Second, the aims of the programmes are typically to change the long-term developmental paths of the participants, but longitudinal studies are often not feasible, and the establishment of long-term preventive effects is difficult. However, even if the obstacles are many, it is still important to explore the efficacy of outreach programmes, as the stakes are extremely high. If left alone, the target population of at-risk youth are likely to develop serious long-term problems, which are not just detrimental to the individual but also very costly to societies.

| Description of the intervention
The intervention in this review is targeted outreach work which may be (but does not have to be) multicomponent programmes in which outreach may be combined with other services. There are different meanings of the concept outreach work throughout Europe and a wide variety of outreach initiatives with different arrangements where outreach may work in one or many ways (Svensson, 2003).
The term outreach work as we will use it in this review is commonly known throughout Scandinavia and is corresponding with detached youth work in England (similar to street work or fieldwork, Korf et al., 1999). Detached outreach work is executed outside any agency setting, is taking place in the community where groups of marginalised youth are known to meet, with the aim of engaging young people who lack any kind of belonging by directing young people to treatment or care services when necessary. It may be based on voluntary efforts, peer groups or professionals, social workers, social pedagogical workers and health workers but the common nature is to meet the young people on their own terms. Outreach work is based on voluntary participation and is an important approach for intervening with hard to reach populations, and identifying their needs in a flexible and responsive manner with no manual-based restrictions.
However, an outreach programme may be associated with a specific service or combination of services offered by one or more organisations targeting a specific population. The services combined with the outreach component could be case management or participation in community programmes or even a continuum of comprehensive services including education, employment, and intensive supervision.
Outreach efforts with services only focusing on nutritional and medical care (e.g., testing for HIV) was excluded.
The comparison population were young people at-risk who are not contacted by the outreach workers and are not encouraged to attend any services.

| How the intervention might work
The primary mechanism of change in outreach work with at-risk youth is to facilitate positive change by gradually building up a sense of trust between the young person and the outreach worker(s) (Svensson, 2003). Characteristically, the aim of the outreach youth worker is to find solutions to young people's problems in their own environment, rather than deciding while sitting behind a desk what they consider best for the person concerned. The goal is always to prevent further marginalisation and encourage social integration (Svensson, 2003).
Theoretically, outreach work may be understood through an empowerment lens. Empowerment theory is both a value orientation for working in the community and a theoretical model for understanding the processes whereby individuals gain access to resources and acquire skills and knowledge enabling them to take advantage of opportunities within the community and to exert control and influence over decisions that affect their lives (Zimmerman, 2002).
As a value orientation empowerment theory proposes that many social problems exist because of unequal distribution of, and access to, resources within the community. The theory further suggests that many individuals are best served by mutual help, helping others or working for their rights rather than having their needs fulfilled by a benevolent professional (Perkins & Zimmerman, 1995;Zimmerman, 2002). What this means is that outreach work is aimed at enabling the at-risk young person to function more autonomously and adaptively within their community rather than just providing a quick fix for their current problems. Empowerment theory proposes that by identifying strengths rather than pointing out and cataloguing risk factors, at-risk youth may become motivated to actively engage in their own positive change. Outreach work may thus also be understood as aimed at promoting resiliency by enabling the young person to make better use of their personal and social resources.
Theoretically a number of protective factors may serve to buffer the adversity a young person might be exposed to. Protective factors at the personal level may include being physically healthy, having a good self-esteem and adaptive coping skills. At the family level protective factors may include having a supportive network of family or friends and at the societal level protective factors may include living in a community with access to support. Thus, outreach work may be seen as drawing on resiliency theory when working to assist the young person in identifying protective factors (Zimmerman et al., 2013). As proposed by Rappaport (1985) social change based on empowerment is proposed to be brought on by a change of both language and conceptions. Instead of perceiving the outreach workers and at-risk young people as 'professionals' and 'clients', empowerment thinking proposes a bidirectional relationship between helpers and participants. In outreach work this means that the outreach workers aim to meet the at-risk youth with a non-judgemental approach characterised by genuine empathy rather than prejudice and victim blaming (Svensson, 2003;Zimmerman, 2002). In addition to meeting the youth with empathy outreach workers strive to become 'culturally competent' which may be defined as the willingness to understand young people from different cultural and social backgrounds and the ability to put oneself in their situation. It also includes the ability and readiness to sympathise with young people subjected to prejudice, social exclusion and stigmatisation, and to approach each young person with respect, open-mindedness and commitment (Svensson, 2003).
As stated in the introduction at-risk youth often come from socio-economically less advantaged and dysfunctional families (Treskon, 2016). At risk youth have often experienced at number of adverse events such as poverty, emotional or physical abuse and neglect, out-of-home placement, living with mentally ill or substance abusing parents and unstable housing situations leading to a lack of continuity in their education. Thus, at-risk youth often lack stable attachment figures and suitable adult role models, which leads to a lack of adaptive life skills and compromises their ability to seek appropriate help within established venues. Early adverse experiences may also lead to a deeply installed mistrust of authorities and thus at-risk youth are often unlikely to seek out help for themselves.
In line with empowerment thinking, outreach programmes seek to meet the young person at their own terms offering them the specific help they need here and now and thus slowly building up a trusting relationship which may be used for future motivational work (Svensson, 2003). Outreach workers aim at offering the young person a positive adult role model and thus provide the young person with the kind of socio emotional support which they often lack.
Sometimes outreach workers may teach the young person basic life skills, such as personal hygiene, offer assistance with homework or writing job applications, paying bills, getting help for substance or alcohol abuse problems and being on time for work or school, or they may accompany the young person to meetings with authority figures, which are fear-inducing in the young person due to their negative past experiences. Furthermore, outreach work may include tutoring programmes, or offer assistance with baby-sitting and housing for socially disadvantaged teenage mothers. What characterises all efforts is that they seek to support and instal a sense of empowerment within the young person which may enable them to master similar challenges in the future in a more adaptive way and to motivate the young person to behaviour changes which may facilitate further social re-integration (Perkins & Zimmerman, 1995;Svensson, 2003;Zimmerman, 2002).
In sum, empowerment theory provides a framework for understanding the mechanisms of change within youth outreach work. The goal of outreach work with at-risk youth is to facilitate positive long-term social change by motivating the young person to become actively engaged. Based on Svensson (2003) the theoretical approach to youth outreach work is based on the following principles: -Distribution of services where youth, subcultural groups, young people at risk and young drug users are present in their own environment.
-To design services based on the needs young people demonstrate and encourage their voluntary participation.
-The outreach work is based on voluntary relations between the youth and the outreach worker. The relation is based on confidence, distinctness and continuity.
-The outreach work is executed on the young people's own terms. | 5 of 28 2.4 | Why it is important to do this review We have located one systematic review on outreach programmes for youth; however, it only included programmes for street-involved youth, a term used by the authors instead of homeless youth (Connolly & Joly, 2012). The participant population was young people aged 12-25, who did not have a permanent place of residence. Another systematic review on homeless youth (between the ages of 12-24 years) focused solely on HIV/acquired immunodeficiency syndrome (AIDS) prevention programmes (Naranbhai et al., 2011).
The searches were performed up to December 2010 and only randomised controlled trials were included.
In the systematic review by Altena et al. (2010), studies published up to 2008 were included if they empirically examined the effectiveness of an intervention for homeless youth. Randomised as well as nonrandomised studies and studies without a control group, that is, beforeafter studies were included. No meta-analysis was performed, only a narrative analysis describing each study and results.
The systematic review by Slesnick et al. (2009), included runaway, shelter, street or drop-in centre recruited youth between the ages of 12-24. In addition to intervention studies, the review also included studies assessing youth outcomes after shelter or drop-in utilisation (i.e., service evaluations) and qualitative studies. No metaanalysis was performed, only a narrative analysis describing each study and results. When the searches were performed is not reported.
In Xiang (2013), studies that examined the effectiveness of interventions to improve substance abuse problems amongst homeless youth between the ages of 12 and 24 were included.
Searches were performed up to April 2012. Only studies that reported data on substance use outcomes were included. Randomised as well as non-randomised studies and studies without a control group, that is, before-after studies were included. No metaanalysis was performed, only a narrative analysis describing each study and results.
Three systematic reviews were found, focusing explicitly on mentoring interventions for youth. Tolan et al. (2008)  Besides being up-to-date, a major difference between these nine systematic reviews and the review we have performed is, that we focused on programmes with a targeted outreach component for youth aged 8-25. Participants need not be homeless (but were eligible if they were), and we only included studies with a control group. All relevant outcome areas were analysed separately in meta-analyses taking into consideration the dependencies between effect sizes.

| Policy relevance
Public as well as private after-school programmes and youth clubs that provide healthy alternatives for youth have been shown to serve as important resources for reducing school failure and youth crime (Parker, 2011). However, it is questionable whether the youth who would benefit most are those who are attracted to and attend such programmes . Outreach work represents an important preventive working approach with the aim of attracting and serving the youth who are very unlikely to participate on their own and who probably need help the most.
Outreach programmes targeting at-risk youth are designed to reach the youth who need help to prevent high-school dropout, crime, drug abuse, and other forms of delinquency. Besides the nonmonetary costs in terms of pain, suffering, and lost quality of life the youth incur themselves, there are potentially large financial costs to society that can be saved. A 1998 study estimated the total costs to society of allowing one youth to leave high school for a life of crime and drug abuse to be somewhere between $1.7 and $2.3 million (Cohen, 1998). There are thus more than one good reason to put more weight on prevention efforts.

| OBJECTIVES
The main objective of this review was to answer the following research questions: What are the effects of outreach programmes on problem/ high-risk behaviour of young people between 8 and 25 years of age living in OECD countries? Are they less likely to experience an adverse outcome such as school failure or drop-out, runaway and homelessness, substance and/or alcohol abuse, unemployment, long-term poverty, delinquency and more serious criminal behaviour? 4 | METHODS 4.1 | Criteria for considering studies for this review

| Types of studies
The proposed project followed standard procedures for conducting systematic reviews using meta-analysis techniques. The systematic review protocol (Filges et al., 2020) was published in December 2020.

1002/cl2.1121.
To summarise what is known about the possible causal effects of outreach, we included all study designs that use a well-defined control group. Non-randomised studies, where outreach has occurred in the course of usual decisions outside the researcher's control, must demonstrate pre-treatment group equivalence via matching, statistical controls, or evidence of equivalence on key risk variables and participant characteristics. These factors were outlined in the protocol, and the methodological appropriateness of the included studies assessed according to a risk of bias model.
The study designs eligible for inclusion in the review were: 1. Controlled trials (where all parts of the study are prospective, such as identification of participants, assessment of baseline, and allocation to intervention, and which may be randomised or nonrandomised), assessment of outcomes and generation of hypotheses (Higgins & Green, 2011).
2. Non-randomised studies (outreach has occurred in the course of usual decisions, the allocation to outreach, and no outreach is not controlled by the researcher, and there is a comparison of two or more groups of participants, that is, at least a treated group and a control group).
Non-randomised studies using an instrumental variable approach were not eligible-see Supporting Information: Appendix 1 (Justification of exclusion of studies using an instrumental variable (IV) approach) for our rationale for excluding studies of these designs.

| Types of participants
Young people between 8 and 25 years of age living in OECD countries, who either have experienced or is at-risk of experiencing an adverse outcome such as school failure or drop-out, runaway and homelessness, substance and/or alcohol abuse, unemployment, longterm poverty, delinquency/criminal behaviour were eligible.
At-risk may be based on such indicators as the young person's level of association with negative peers (e.g., negative attitudes towards school and poor educational outlook, gang members, etc.), hanging out on the streets or in gang neighbourhoods, poor academic history, coming from a highly distressed or crisis ridden, low income family in a racially/ethnically segregated neighbourhood, and prior involvement in illegal and delinquent activities.
Studies where the majority of participants are between 8 and 25 years of age were not eligible.

| Types of interventions
The intervention in this review are targeted outreach work which may be combined with other services. There are different meanings of the concept outreach work throughout Europe (Svensson, 2003). The term outreach work as we will use it in this review is commonly known throughout Scandinavia and is corresponding with detached youth work in England (similar to street work or fieldwork, Korf et al., 1999).
Detached outreach work is executed outside any agency setting, is taking place in the community where groups of marginalised youth are known to meet, with the aim of engaging young people who lack any kind of belonging, and directing young people to treatment or care services when necessary. An outreach programme may be associated with a specific service or combination of services offered by one or more organisations targeting a specific population. The services combined with the outreach component could be case management or participation in community programmes or even a continuum of comprehensive services including education, employment, and intensive supervision.
Outreach efforts with services only focusing on nutritional and medical care (e.g., testing for HIV) were excluded.
The comparison population were young people at-risk who are not contacted and encouraged by the outreach workers to attend any services.

| Types of outcome measures
The primary outcome was problem/high-risk behaviour, as the overall review question is to evaluate current evidence on FILGES ET AL. | 7 of 28 outreach programmes' effects on problem/high-risk behaviour for young people who have experienced or are at risk of experiencing an adverse outcome. We sought evidence on how to best reduce or eliminate problem/high-risk behaviour, as problem/high-risk behaviour is understood as the young people's primary problem.
All measures were included, that is, we did not require that measures have been standardised on a different population.

| Primary outcomes
The primary focus was on measures of problem/high-risk behaviour, such as delinquency/criminal behaviour, drug and alcohol use, high levels of externalising problems, school failure, sexual risk taking, gang involvement/membership, poverty, unemployment, runaway and homelessness.

| Secondary outcomes
A secondary focus was on measures of social and emotional outcomes, such as internalising symptoms (anxiety, depression), self-identity, interpersonal relations and social awareness.

| Adverse outcomes
Any adverse effects of interventions were included as an outcome including a worsening of outcome on any of the included measures.

Duration of follow-up
We planned to include outcomes measured during and after intervention as well as follow-up at any given point in time.

Types of settings
Detached outreach work is executed outside any agency setting, is carried out in the community where groups of marginalised youth are known to meet, with the aim of engaging young people who lack any kind of belonging, and attracting young people to treatment or care services when necessary.
Distribution of outreach services thus takes place where youth, subcultural groups, young people at risk and young drug users are present in their own environment.
Furthermore, outreach services delivered in any format meaning were eligible, that is, services that are delivered at an individual level (that includes conversation, adult contacts, following up and being available), at a group level (the outreach worker relates to different youth groups and gangs, and initiates in-group activities) and finally local community work (such as finding places for the young people to spend their spare-time, contact and collaboration with other youth workers and between voluntary and public organisations when that is suitable).

| Search methods for identification of studies
We implemented a wide range of search methods and strategies to maximise coverage of relevant references, while simultaneously attempting to reduce different types of bias related to publication and dissemination systems. The different strategies and methods will be presented below.

Selection of bibliographical databases
We selected bibliographical databases that cover journals from different academic disciplines relating to the topic of the review. We also selected databases with a general academic scope, to ensure coverage beyond the expected academic fields. We selected the follow databases: The database searches were performed in September 2020.

Example of a search string
The search strings were modified according to the search interface, syntax and subject terms for each of the above standing databases.
All database searches are documented in Supporting Information: Appendix 2.
Description and rationale for search terms and facets, and sensitivity of the search string The search string was designed to balance sensitivity and precision.
The search string contains two aspects related to the inclusion criteria of the review. To keep the search string sufficiently sensitive, we searched each aspect in either title, abstract or subject terms.

Limitations of the search string
The supplemental and grey literature sources included American, Swedish, Danish, and Norwegian sources, but did not include specific other regional sources (Canadian, Australian, British) which may be a limitation of the review. We did not implement any language or year restrictions to the searches on bibliographical databases.

| Searching other resources
The searches on other resources and for unpublished literature was done between the 13/10/2021 and 30/11/2021. We searched a range of web-based resources to identify references that where either unpublished, not in English, or both. Terms used to search other resources were based on the general search strategy.
Combinations of terms such as outreach with terms for the population (i.e., youth or at-risk) were utilised. All of these searches can be seen in Supporting Information: Appendix 2.
Due to the language restrictions of the review team, we selected

Hand searches
We implemented hand searches in key journals to identify references that were poorly indexed in the bibliographical databases, as well as covering references that was published in a journal, but not yet indexed in the bibliographical databases during the search process.

Citation-tracking and snowballing methods
To identify both published studies and grey literature we utilised citation-tracking/snowballing strategies. Our primary strategy was to citation-track related systematic-reviews and meta-analyses. The review team also checked reference lists of included primary studies for new leads.

Contact to experts
By e-mail during September 2021, we contacted international experts to identify unpublished and ongoing studies.

| Criteria for determination of independent findings
To account for possible statistical dependencies, we examined a number of issues: we assessed whether individuals had undergone multiple interventions, whether there were multiple treatment groups and whether several studies were based on the same data source. Multiple studies using the same sample of data There were no studies using the same sample of data. Extracted numerical and descriptive data, and the risk of bias assessments described in the next section, can be found in the supplementary documents.

| Assessment of risk of bias in included studies
We assessed the risk of bias in randomised studies using Cochrane's revised risk of bias tool, ROB 2 (Higgins et al., 2019).
The tool is structured into five domains, each with a set of signalling questions to be answered for a specific outcome. The five domains cover all types of bias that can affect results of randomised trials.
The five domains for individually randomised trials are: (1) bias arising from the randomisation process; (2) bias due to deviations from intended interventions (separate signalling questions for effect of assignment and adhering to intervention); (3) bias due to missing outcome data; (4) bias in measurement of the outcome; (5) bias in selection of the reported result. for cluster randomised parallel-group trials). In the cluster randomised template however, only the risk of bias due to deviation from the intended intervention (effect of assignment to intervention; intention to treat ITT) is present and the signalling question concerning the appropriateness of the analysis used to estimate the effect is missing.
Therefore, for cluster randomised trials we only used the signalling questions concerning the bias arising from identification or recruitment of individual participants within clusters from the template for cluster randomised parallel-group trials; otherwise we used the template and signalling questions for individually randomised parallelgroup trials.
We assessed the risk of bias in non-randomised studies, using the model ROBINS-I, developed by members of the Cochrane Bias Methods Group and the Cochrane Non-Randomised Studies Methods Group (Sterne et al., 2016a). We used the latest template for completion (currently it is the version of 19 September 2016).
The ROBINS-I tool is based on the Cochrane RoB tool for randomised trials, which was launched in 2008 and modified in 2011 (Higgins et al., 2011).
The ROBINS-I tool covers seven domains (each with a set of signalling questions to be answered for a specific outcome) through which bias might be introduced into non-randomised studies: (1) bias due to confounding (2) bias in selection of participants (3) bias in classification of interventions (4) bias due to deviations from intended interventions; (5) bias due to missing outcome data; (6) bias in measurement of the outcome; (7) bias in selection of the reported result. In the case of a RCT, where there is evidence that the randomisation has gone wrong or is no longer valid, we planned to assess the risk of bias of the outcome measures using ROBINS-I instead of ROB 2. Examples of reasons for assessing RCTs using the ROBINS-I tool may include studies showing large and systematic differences between treatment conditions while not explaining the randomisation procedure adequately suggesting that there was a problem with the randomisation process; studies with large-scale differential attrition between conditions in the sample used to estimate the effects; or studies selectively reporting results for some part of the sample or for only some measured outcomes. In such cases, differences between the treatment and control conditions are likely systematically related to other factors than the intervention and F I G U R E 1 Flow diagram FILGES ET AL.
| 11 of 28 the random assignment is, on its own, unlikely to produce unbiased estimates of the intervention effects. Therefore, as ROBINS-I allow for an assessment of for example confounding, we believe it is more appropriate to assess effect sizes from studies with a compromised randomisation using ROBINS-I than ROB 2. We reported this decision as part of the risk of bias assessment of the outcome measure in question (one study and all outcomes measured in this study was moved from ROB 2 to ROBINS-I). As other effect sizes assessed with ROBINS-I, the effect sizes could have received a 'Critical' rating and thus be excluded from the data synthesis.
We stopped the assessment of a non-randomised study outcome as soon as one domain in the ROBINS-I was judged as 'Critical'.
'Serious' risk of bias in multiple domains in the ROBINS-I assessment tool may lead to a decision of an overall judgement of 'Critical' risk of bias for that outcome, and it will be excluded from the data synthesis.

Confounding
An important part of the risk of bias assessment of non-randomised studies is consideration of how the studies deal with confounding factors. Systematic baseline differences between groups can compromise comparability between groups. Baseline differences can be observable (e.g., age and gender) and unobservable (to the researcher; e.g., motivation and 'ability'). There is no single nonrandomised study design that always solves the selection problem. A major difficulty in estimating causal effects of outreach work is the potential endogeneity of the young individual's life circumstance that leads to the decision of the outreach worker to reach out to that particular young person and if not accounted for it will yield biased estimates.
As there is no universal correct way to construct counterfactuals for non-randomised designs, we looked for evidence that identification was achieved, and that the authors of the primary studies In addition to unobservables, we had identified the following observable confounding factors to be most relevant: age, gender and risk indicators as described in section Type of participants. In each study, we assessed whether these factors had been considered, and in addition we assessed other factors likely to be a source of confounding within the individual included studies.

Importance of prespecified confounding factors
The motivation for focusing on age, gender and risk indicators is given below.
The prevalence of different types of behavioural and psychological problems, coping skills, cognitive and emotional ability vary throughout a child's development through puberty and into adulthood (Cole et al., 2005), and therefore we consider age to be a potential confounding factor. Furthermore, there are substantial gender differences in behaviour problems, coping and risk of different types of adverse outcomes which is why we also include gender as a potential confounding factor (Card et al., 2008;Hampel & Petermann, 2005;Hart et al., 2007).
Pre-treatment group equivalence of risk indicators is indisputable an important confounder as young people in stable life circumstances, typically are not at risk of developing the range of problems we will consider in this review. Therefore, the accuracy of the estimated effects of outreach programmes will depend crucially on how well the risk indicators are controlled for.

Effect of primary interest and important co-interventions
We were mainly interested in the effect of starting and adhering to the intended intervention, that is, the treatment on the treated (TOT) effect. The risk of bias assessments was therefore performed in relation to this specific effect.
As the intervention is outreach to young people who are very unlikely to seek out help for themselves, we could not think of any important differences in additional interventions ('co-interventions') between intervention groups that could bias the estimated effect.

Assessment
At least two review authors independently assessed the risk of bias for each relevant outcome from the included studies. We discussed all initial disagreements and were able to reach a consensus in all cases. We report the risk of bias assessment in risk of bias tables for each included study outcome in a supplementary document.

Continuous outcomes
All but two outcomes (housing and NEET status) were continuous measures. We calculated effects sizes with 95% confidence intervals, where means and standard deviations were available, or alternatively from mean differences and standard deviations of the mean (whichever were available), using the methods suggested by Lipsey and Wilson (2001). If not enough information was available, we requested this information from the principal investigators. Hedges' g was used for estimating standardised mean differences (SMD).

Dichotomous outcomes
For the three dichotomous outcomes (housing, NEET status and gang membership), we used odds ratios with 95% confidence intervals.

| Unit of analysis issues
There were no studies where the unit of allocation differed from the unit of analysis.

| Dealing with missing data
Missing data and attrition rates was assessed in the included studies; see section Assessment of risk of bias in included studies.
Where studies had missing summary data, such as missing standard deviations, we requested this information from the principal investigators. We contacted Professor Kidd who kindly forwarded our request to researcher Scott Leon who provided the necessary information. We also contacted Professor Amy

Arbreton and Wendy McClanahan concerning two studies
published by the now closed Public/Private Ventures. They kindly replied to our request but unfortunately the information we were seeking no longer exist.

| Assessment of heterogeneity
Heterogeneity among primary outcome studies was assessed with χ 2 (Q) test, and the I 2 , and τ 2 statistics (Higgins et al., 2003). Any interpretation of the χ 2 test was made cautiously on account of its low statistical power.

| Assessment of reporting biases
Reporting bias refers to both publication bias and selective reporting of outcome data and results. Here, we state how we planned to assess publication bias.
We planned to use funnel plots for information about possible publication bias however we did not find sufficient studies (Higgins & Green, 2011).

| Data synthesis
Meta-analysis of outcomes were conducted on each metric (as outlined in section 'Types of outcomes measures') separately.
Studies that were coded Critical risk of bias were not included in the data synthesis.
All analyses were inverse variance weighted using random effects statistical models that incorporate both the sampling variance and between study variance components into the study level weights. Random effects weighted mean effect sizes were calculated using 95% confidence intervals.
We provided a graphical display (forest plot) of effect sizes.

| Subgroup analysis and investigation of heterogeneity
There were not enough studies to perform moderator analyses.

| Sensitivity analysis
There were not enough studies to perform sensitivity analyses.

Treatment of qualitative research
We did not plan to include qualitative research.  Table 1) and 406 studies were ordered, retrieved, and screened in full text. Of these, 390 did not fulfil the screening criteria and were excluded. We included a total of 16 studies in the review. The references are listed in section References to included studies. In Table 2 we show the total number of studies, that met the inclusion criteria for this review. The first column shows the total number of studies grouped by country of origin. The second column shows the number of these studies that did not provide enough data to calculate an effect estimate. The third column gives the number of studies that were coded with Critical risk of bias. The last column gives the total number of studies used in the data synthesis.

| Included studies
Eight studies were judged overall Critical risk of bias (see supplementary documents for the detailed risk of bias assessments).
In accordance with the protocol, we excluded studies rated overall Critical risk of bias items from the data synthesis on the basis that they would be more likely to mislead than inform. Three studies did not provide enough information enabling us to calculate an effect size and standard error or did not provide enough information to assess risk of bias. All studies are listed in Table 3 along with the reason why the study was not used in the data synthesis.
The main characteristics of the five studies used in the data synthesis are shown in Table 4. Ethnicity of outreach participants was reported in only four studies and the average percent of white was 43% with great variation, ranging from 20% to 72%. The target population was homeless youth in three studies and youth at risk of failing to appear for court hearings and youth at risk of gang membership in one study each. The services provided in connection with outreach were mental health (one study), peer support (one study), case management (two studies) brief motivational intervention (two studies) and after-school activities (one study). Note that more than one activity could be provided in connection with outreach.

| Excluded studies
In addition to the 16 studies that met the inclusion criteria for this review, 17 studies at first sight appeared relevant but did not meet our criteria for inclusion. The studies and reasons for exclusion are given in Table 5. More than a third (seven studies) were excluded because the intervention analysed was not outreach as defined in this review. Other reasons were lack of control group (three studies), intervention not targeted to youth (two studies) and no analysis on an individual level were performed (five studies).

| Risk of bias in included studies
The risk of bias coding for each of the 16 studies and their outcomes is shown in a supplementary document.
Four studies reported on randomised trials, all individually randomised trials. Table 6 shows a summary of the risk of bias associated with the randomised studies.
Three studies reported an appropriate method of randomisation and to some extent showed or discussed baseline imbalances on the pre-specified confounders. We rated all three studies Some concerns on the Randomisation Process item as they all had some issues with the balance on the pre-specified confounders.
One study did not report the randomisation method but most likely it was concealed and there were no imbalances on the prespecified confounders. This study was also rated Some concerns on the Randomisation Process item. On the Deviations from intervention item, all four studies were rated some concerns, mainly due to lack of blinding. Concerning missing outcome data, one study had no issues, and we rated it Low risk of bias, three studies were rated Some concerns. All four studies were rated Some concerns on the Measurement of Outcome item, mainly due to lack of blinding. One study was rated Low risk of bias on the Selection of Reported Results item, the remaining were rated Some concerns as there was no a priori analysis plan and an insufficient reporting of outcomes. Overall, none of the studies were rated Low risk of bias, they were all rated some concerns overall.
Unfortunately the study Herrera et al. (2013)  The assessment of one study (Walker et al., 2019) was rated using the ROBINS-I tool, as even though participants were randomised (although not using an appropriate method), youth assigned to the comparison group from a previous evaluation (approximately 1 year before the current study) was included.
This compromised the randomisation to an extent that it was most appropriate to assess the risk of bias using the ROBINS-I tool.
The remaining 11 studies used non-randomised designs, 1 study  reported on 2 different interventions including different individuals so in total 13 interventions were rated using the ROBINS-I tool. Table 7 shows a summary of the risk of bias associated with the non-randomised studies. As stated in the protocol, we stopped the assessment of a non-randomised study outcome when it was rated 'Critical' on any of the items, therefore not all studies are rated on all domains. One study (McClanahan et al., 2012) T A B L E 5 Studies excluded with reason Study Reason for exclusion Augimeri et al. (2007) The SNAPTM under 12 outreach project (ORP) is a manualized 12-week outpatient program with five primary components, not outreach Campie et al. (2017) City level outcomes only Domina (2009) It is a college preparation program for the disadvantaged, and the study provides little information on the sort of outreach activities available to the students and schools that participate in outreach, some of them may be talent programs and some offer offer yearround college advising and information, academic counselling, tutoring services, and special full-day summer programs.
Georgiades (2003) Not outreach as defined in this review Green et al. (2011) Not outreach as defined in this review Guo and Slesnick (2017) Both groups receive outreach followed by drop-in or shelter Hureau (2016) Not targeted to youth but to gangs in general, age is not mentioned or reported at all | 17 of 28 stated that a detailed description of the analysis was summarised in the Technical Appendix, which unfortunately is not available. Thus, we could not assess risk of bias for this study, as there was very little (close to nothing) description in the main text. We contacted the author, but unfortunately she does not have the technical appendix and the publishing institution (P/PV) closed in 2012. The study could therefore not be rated.
Eight of the non-randomised studies were rated Critical risk of bias on the Overall judgement item corresponding to a risk of bias so high that the findings should not be considered in the data synthesis. The overall Critical risk of bias rating was mainly due to issues on the Confounding bias item; four were rated Critical risk of bias on this item; that is, they failed to establish a comparison group that was balanced on important confounders and further either did not control for any confounders or Three studies (reporting on four interventions) were rated Serious risk of bias overall. Unfortunately, the study  reporting on two interventions did not report data that permitted calculation of an effect size and standard error. We contacted the authors but unfortunately, the data is no longer available as the publishing institution (P/PV) closed in 2012. This left only two non-randomised studies to be used in the data synthesis.
Of the four interventions not rated Critical risk over bias overall, all had serious issues on this item. On the Selection bias item three were rated Moderate risk of bias and one was rated Serious risk of bias. All were rated Low risk of bias on the Classification item; three were rated Low risk of bias on the Deviation item and one was rated Moderate. On the missing data item two were rated Low risk of bias, one was rated Moderate and one was rated Serious risk of bias. On the measurement item, one was rated Low risk of bias and three were rated Moderate risk of bias. All four were rated Moderate risk of bias on the Selection of Reported Results mainly because there was no a priori analysis plan.

| Synthesis of results
Five studies were not rated Critical risk of bias and reported data that permitted calculation of an effects size and standard error and could thus be used in the meta-analysis.
A large variety of different outcomes were reported in the studies (e.g., drug use, abstinence, housing, mental and physical health).
To carry out a meta-analysis, every study must have a comparable effect size. We synthesise effects separately by type of outcome (conceptual outcomes as outlined in section 'Types of outcomes measures') and time point (end of intervention and follow up). Unfortunately each type of outcome was only reported in a small subset of studies (in many cases in only one single study). Thus, each meta analysis contains a very small number of effect sizes, at most two. The studies included in the meta-analyses contribute only a single effect size to each analysis.
All continuous outcomes (effect sizes measured as Hedges g) were coded such that a larger effect size indicated better outcomes for the treated group. All binary outcomes (reported as odds ratio) were likewise coded such that a larger effect size indicated better outcomes for the treated group.

Primary outcomes
Two studies analysed the effect of outreach on three different substance uses in last 30 days: drug (other than marijuana), marijuana and alcohol. Both studies reported on T A B L E 7 Summary risk of bias non-randomised studies Note: Twelve studies were rated, one with two interventions, that is, 13 ratings in total, some rated differently on outcomes but best rating included here. outcomes at two time points; 1 month post-baseline and 3 months post-baseline.
Drug, other than marijuana. The random effects weighted standardised mean difference at 1 month post-baseline was 0.0 (95% confidence interval [CI]: −0.29 to 0.29) and not statistically significant. The forest plot is displayed in Figure 2. There was a very small amount of heterogeneity between the studies; the estimated τ 2 was 0.01, Q = 1.27, df = 1 and I 2 was 21% as displayed in Figure 2.
The random effects weighted standardised mean difference at 3 months post-baseline was 0.07 (95% CI: −0.18 to 0.33) and not statistically significant. The forest plot is displayed in Figure 3. There was no heterogeneity between the studies; the estimated τ 2 was 0.00, Q = 0.41, df = 1 and I 2 was 0% as displayed in Figure 3.
Marijuana. The random effects weighted standardised mean difference at 1 month post-baseline was 0.04 (95% CI: −0.21 to 0.29) and not statistically significant. The forest plot is displayed in Figure 4. There was no heterogeneity between the studies; the estimated τ 2 was 0.00, Q = 0.39, df = 1 and I 2 was 0% as displayed in Figure 4.
The random effects weighted standardised mean difference at 3 months post-baseline was −0.03 (95% CI: −0.29 to 0.22) and not statistically significant. The forest plot is displayed in Figure 5. There | 19 of 28 was no heterogeneity between the studies; the estimated τ 2 was 0.00, Q = 0.37, df = 1 and I 2 was 0% as displayed in Figure 5.
Alcohol. The random effects weighted standardised mean difference at 1 month post-baseline was 0.05 (95% CI: −0.21 to 0.30) and not statistically significant. The forest plot is displayed in Figure 6. There was no heterogeneity between the studies; the estimated τ 2 was 0.00, Q = 0.23, df = 1 and I 2 was 0% as displayed in Figure 6.
The random effects weighted standardised mean difference at 3 months post-baseline was −0.17 (95% CI: −0.43 to 0.09) and not statistically significant. The forest plot is displayed in Figure 7. There was no heterogeneity between the studies; the estimated τ 2 was 0.00, Q = 0.28, df = 1 and I 2 was 0% as displayed in Figure 7.
Other primary outcomes. In addition, a number of outcomes were reported in a single study only. The outcomes were measures on housing situation, NEET status, gang membership, externalising problems and delinquency/criminal behaviour. The effect sizes and 95% CIs are reported in Table 8.  Thompson and Jason (1988) Comparing names with gang rosters provided by gang members involved with BUILD's remediation program.

| Overall completeness and applicability of evidence
We included in total five studies in the data synthesis and of these, a maximum of two studies reported the same outcome and could be used in a specific meta-analysis. This number is lower than the number of studies (16) meeting the inclusion criteria. The reduction was caused by three different factors.
Eight studies were judged to have a Critical risk of bias and, in accordance with the protocol, we excluded these from the data synthesis on the basis that they would be more likely to mislead than inform. One study provided very little (close to nothing) information on the method of analysis in the main text and referred to a technical appendix for this information.
We contacted the author, but unfortunately she does not have the technical appendix and the publishing institution (P/PV) closed in 2012. The study could therefore not be rated and could not be used in the data synthesis. Finally, two studies (reporting on three different interventions) did not report effect estimates or provide data that would allow the calculation of an effect size.
If all the included studies had provided an effect estimate with lower risk of bias, the final list of useable studies in the data synthesis would have been larger, which again would have provided a more robust literature on which to base conclusions.
All studies used in the data synthesis were from the USA and Canada. This narrow geographical coverage is a clear limitation of the review.
Long term follow-up analyses were not possible. This is also a clear limitation of the review.
It was not possible to examine the impact of the moderators nor sensitivity analyses for each outcome to check whether the obtained results were robust across study design and methodological quality. | 21 of 28 6.3 | Quality of the evidence The majority of studies (12) used non-randomised designs, and four were randomised trials. Overall the risk of bias in the included studies was high. Among the non-randomised studies only three studies (reporting on four interventions) were not rated Critical risk of bias (in addition, one study provided too little information to be rated). The level 'Critical' means: the study (outcome) is too problematic in this domain to provide any useful evidence on the effects of intervention, and it is excluded from the data synthesis.
None of the randomised trials were overall rated low risk of bias, they were all assessed to have some concerns overall.
We examined the risk of bias using Cochrane's revised The quality of the evidence in this review was enhanced by excluding studies assessed to be at critical risk of bias using the ROBINS-I tool from the data synthesis. We believe this process excluded those studies that are more likely to mislead than inform.
With two studies contributing effect sizes for one outcome (although reported at two different time points) it is of little use to discuss overall consistency in the direction and magnitude of effects and heterogeneity between studies.

| Potential biases in the review process
We performed a comprehensive electronic database search, combined with grey literature searching, and hand searching of key journals. All citations were screened in teams by two independent screeners from the review team (TPC, MCTM., FSB., and FLWS), and one review author (TF) assessed all included studies against inclusion criteria.
We believe that all the publicly available studies on the effect of outreach on young people's problem/high-risk behaviour and social and emotional outcomes up to the censor date were identified during the review process.
However, six references were not obtained in full text.
We were unable to comment on the possibility of publication bias as at most two studies was included in the same meta-analysis.
Thus, we cannot rule out that there are still some missing studies, which were not published or made public.
We believe that there are no other potential biases in the review process as two members of the review team (MCTM, FLWS) independently coded the included studies. Any disagreements were resolved by discussion. Further, decisions about inclusion of studies were made by the two teams of each two members of the review team (TPC, MCTM, FSB, FLWS) and one review author (TF).
Assessment of study quality and numeric data extraction was made by one review author (TF) and each study was checked by another review author (NTD).

| Agreements and disagreements with other studies or reviews
One systematic review on outreach programmes for youth; only including programmes for street-involved youth, a term used by the authors instead of homeless youth, was found (Connolly & Joly, 2012 | 23 of 28 ethnicity, risk factors and household characteristics The risk of bias due to confounding would be judged to be of less concern had the primary study authors controlled for these factors. As the data already are gathered it is recommended that analyses controlling for important confounding factors are carried out using these data. Unfortunately two of the included studies (one randomised and one non-randomised) did not provide data that permitted the calculation of an effect size and standard error (and attempt to achieve them was fruitless) and could therefore not be used in the data synthesis.
Given the limited number of rigorous studies available from countries other than the USA and Canada, it would be natural to consider conducting randomised controlled trials even though it might be argued that it is difficult for the population of interest.
However, depending on the specific target population, an appropriate control group could be obtained in several ways. If the target population is homeless youth, treatment and control youth may be recruited from drop-in centres, shelters or other agencies serving homeless youth and to include an even broader range of youth street sampling methods could be used. All these techniques were for example combined in the randomised controlled study (Peterson et al. (2006)). If the target population is school children, schools could be the unit of randomisation or, as in the study Herrera et al. (2013) evaluating mentoring through the Big Brothers Big Sisters programme, In this study they reached at-risk youth in a number of ways, including collaborations with schools, partnerships with social service agencies, participation in community activities and events, word of mouth, and other media/communications strategies. Interested youth were then randomised to treatment and a wait list control group.
Such adapted trials in other countries than the USA and Canada would have the potential of making useful contributions to the outreach effectiveness literature if due consideration is made to the strengths and weaknesses of the studies found in this review. Thus, besides specific attention would have to be paid to stringency in terms of conducting a well-designed randomised trial with low risk of bias as well as ensuring that the sample sizes are large enough to enable sufficient power, the trials should also pay attention to reporting relevant outcomes with sufficient details for them to be included in an inverse variance weighted meta-analysis of standardised effect sizes. Further, trials performed in countries with access to administrative data about the participant's school, housing, employment and health outcomes (e.g., Denmark) would enable the investigator to report on long-term effects of the intervention.

DECLARATIONS OF INTEREST
There are no potential conflicts of interest.

PRELIMINARY TIMEFRAME
Approximate date for submission of the systematic review will be no longer than two years after protocol approval.

PLANS FOR UPDATING THIS REVIEW
Trine Filges will be responsible for updating the review and updates can be expected each second year.

DIFFERENCES BETWEEN PROTOCOL AND REVIEW
We planned to add a critical level of risk of bias to the RoB 2 tool with the same meaning as in the ROBINS-I tool; that is, the study (outcome) is too problematic in this domain to provide any useful evidence on the effects of intervention, and it is excluded from the data synthesis. However, after publication of the protocol we became aware (through correspondence with Professor Julian Higgins) that our add-on (of a 'Critical' risk of bias level) to the ROB 2 tool is in breach of the Creative Commons licence for RoB 2.
We therefore made the following change to the application of the ROB 2 tool: In the case of a RCT, where there is evidence that the randomisation has gone wrong or is no longer valid, we planned to assess the risk of bias of the outcome measures using ROBINS-I instead of ROB 2. Examples of reasons for assessing RCTs using the ROBINS-I tool may include studies showing large and systematic differences between treatment conditions while not explaining the randomisation procedure adequately suggesting that there was a problem with the randomisation process; studies with large scale differential attrition between conditions in the sample used to estimate the effects; or studies selectively reporting results for some part of the sample or for only some measured outcomes. In such cases, differences between the treatment and control conditions are likely systematically related to other factors than the intervention and the random assignment is, on its own, unlikely to produce unbiased estimates of the intervention effects. Therefore, as ROBINS-I allow for an assessment of for example confounding, we believe it is more appropriate to assess effect sizes from studies with a compromised randomisation using ROBINS-I than ROB 2. We reported this decision as part of the risk of bias assessment of the outcome measure in question (one study and all outcomes measured in this study was moved from ROB 2 to ROBINS-I). As other effect sizes assessed with ROBINS-I, the effect sizes could have received a 'Critical' rating and thus be excluded from the data synthesis.

Search strategy deviations from protocol
We searched the bibliographical database SocIndex.
In January 2021, the Danish National

Internal sources
• No sources of support provided External sources • VIVE Campbell, Denmark